Modern manufacturing processes sometimes require precise stoichiometric ratio of chemical elements during particular manufacturing phases. To achieve these precise ratios, different process gases may be delivered into a process chamber during certain manufacturing phases. A gas panel may be used to deliver these process gasses to a process tool with one or more chambers or reactors. A gas panel is an enclosure containing one or more gas pallets dedicated to deliver process gases to the process tool. The gas panel is in turn composed of a group of gas pallets, which is itself composed of a group of gas sticks.
A gas stick assembly may contain several discrete components such as an inlet fitting, manual isolation valve, binary controlled pneumatic isolation valves, gas filters, pressure regulators, pressure transducers, inline pressure displays, mass flow controllers and an outlet fitting. Each of these components is serially coupled to a common flow path or dedicated channel for one particular process gas. A manifold and a valve matrix channel the outlet of each gas stick to the process chamber.
To achieve a certain stoichiometric ratio a process tool controller asserts setpoints to the mass flow controllers, and sequences the valve matrices, associated with certain gas sticks. The indicated flow value is output by the mass flow controller of each gas stick and monitored by the process tool controller.
A mass flow controller (MFC) is constructed by interfacing a flow sensor and proportioning control valve to a control system. The flow sensor is coupled to the control system by an analog to digital converter. The control valve is driven by a current controlled solenoid valve drive circuit. A mass flow measurement system is located upstream of the control valve. The control system monitors the setpoint input and flow sensor output while refreshing the control valve input and indicated flow output. The closed loop control algorithms executed by the control system operate to regulate the mass flow of process gas sourced at the inlet fitting through the proportioning control valve and outlet fitting such that the real-time difference or error between the setpoint input and indicated flow output approaches zero or null as fast as possible with minimal overshoot and as small a control time as possible. A critically damped response characteristic is desired. Furthermore, the mass flowing into the inlet fitting is desired to be equivalent to the mass flowing from the outlet fitting.
The mass flow sensor is coupled to the MFC flow path using a bypass arrangement along a partial restriction in the flow path that ensures laminar flow in the flow measurement portion of the MFC. The thermal sensor samples only a portion of gas that flows from the inlet fitting through the control valve and from the outlet fitting. A calibration and validation process is applied to the completed mass flow controller assembly to correlate the digitized value of sampled gas flow to a primary mass flow standard. The control system may execute these programmable curve fitting algorithms to apply the correlation such that the mass flow of the process gas is accurate and linear.
This thermal mass flow sensor is constructed by applying heated coils to a capillary tube. The coil material and method of construction are chosen such that the sensor will function as a resistance temperature device or RTD. In an RTD process sensor, a change in resistance maybe proportional to a change in temperature. The heater coils complete an electronic circuit which is designed to precisely excite or energize the coils as well as detect changes in the resistance of the coils. One embodiment of a thermal mass flow sensor has two coils, upstream and downstream. Mass flow through the capillary tube will transfer heat from the upstream coil to the downstream coil as a function of the heat capacity of the gas species flowing through the capillary tube. The downstream coil resistance will change in proportion to the mass flow of the gas species source connected to the inlet fitting of the mass flow controller.
However, MFCs of this type, and their control algorithms, may be particularly sensitive to pressure fluctuation in the process gases and may indicate false flow conditions. Upstream pressure disturbances are caused by the transient stability of discrete pressure regulators located upstream of the MFC inlet fitting or perturbations in the upstream pressure source. False flow conditions occur when a pressure gradient exists within the volume of the MFC fluid path, specifically in the volume that exist downstream of the thermal sensor and upstream of the control valve. Both types of disturbances are a function of the capacity of the gas source, impedance or conductance of the gas delivery system and abrupt transitions in gas flow.
Unfortunately, typical techniques for enhancing the bandwidth of the thermal sensor employed by MFCs inject high frequency components into the indicated flow signal that do not reflect the true value of the actual mass flow exiting the outlet fitting of the mass flow controller during upstream pressure disturbances. The magnitude of the temporary error in flow indication is a function of the volume in the flow path that is downstream of the thermal flow sensor and upstream of the control valve associated with the MFC. The compensated thermal sensor output measures mass flow upstream of the control valve. The real-time position of the throttling control valve is computed by the closed loop control algorithm executed by the control system. As the pressure in this volume changes, the compensated output of the thermal sensor changes. The control system reacts to a change in sensed mass flow by throttling the valve to reduce the error between the setpoint value and the indicated flow value to zero. An error term equivalent to zero assumes that the mass flow rate of actual process gas flowing into the inlet fitting is equivalent to actual process gas flowing from the outlet fitting. This temporary perturbation in indicated flow and actual process gas flow can result in poor transient or steady state stability that can cause wafer damage, tool alarms or unscheduled downtime.
Thus, there is a need for systems and methods for a mass flow controller which minimize false flow conditions and display a reduced sensitivity to pressure transients.